Boron penetration to suppress short channel effect in P-channel device

Abstract

A method for forming a set of p-channel devices with enhanced n-doping and penetration of boron into the channel region between the source and drain regions, thereby creating channel length independent p-channel threshold voltage behavior. Long channel and short channel transistors have approximately equal threshold voltages as (a) short channel effect is reduced with increased n-doping in short channel transistors (where boron penetration has little effect), and (b) the effects of boron penetration and increased n-doping are offset in longer channel transistors.

What is claimed is: 
     
       1. A method for controlling the threshold voltage for a set of p-channel devices formed on a semiconductor substrate, in which the devices have different channel lengths, comprising the steps of: (a) providing an n-doped semiconductor substrate, wherein the n-doping is sufficient to control the threshold voltage within 0.3 volts for p-channel devices with channel lengths of less than 0.6 μm;   (b) forming a set of p-channel devices with gate regions having different channel lengths, said lengths varying from less than 0.6 μm to at least 3 μm; and   (c) causing boron to penetrate into the gate regions of the p-channel devices, whereby the absolute values of the threshold voltage for the devices decreases with increasing channel length, thereby offsetting the effect of n-doping of the substrate for long channel lengths and creating a set of p-channel devices having said controlled threshold voltage behavior for channel lengths from less than 0.6 μm to channel lengths of greater than 3 μm, wherein the threshold voltage is defined as 85% of the saturation voltage.   
     
     
       2. The method of claim 1, wherein the threshold voltage is controlled within 0.3 volts for devices having channel lengths from less than 0.3 μm to greater than 10 μm. 
     
     
       3. The method of claim 1, wherein the n-doping level is in the range of 10 17  cm -3  to 10 18  cm -3 . 
     
     
       4. The method of claim 3, wherein the boron implantation dosage is in the range of 10 15  cm -2  to 10 16  cm -2 . 
     
     
       5. The method of claim 2, wherein the n-doping level is in the range of 10 17  cm -3  to 10 18  cm -3 . 
     
     
       6. The method of claim 5, wherein the boron implantation dosage is in the range of 10 15  cm -2  to 10 16  cm -2 .